Bulldozer design compromises offer mixed bag for desktop use

AMD's first group of Bulldozer-based CPUs, the FX series, have been released and thoroughly benched. The approach behind Bulldozer is what AMD has termed a "third way" between traditional multicore and simultaneous multithreading, which should offer some performance advantages in highly threaded workflows that keep instructions pumping through its 256-bit wide FPUs and doubled-up integer units. But that third way doesn't seem to offer much of a performance or efficiency advantage for many common desktop tasks.

We took a look at thorough testing done by AnandTech, Tech Report, and Tom's Hardware, and recommend giving those reviews a read if you're considering a Bulldozer CPU for your next machine. We'll give a high-level summary here, noting some areas where Bulldozer will shine best and where it falls flat.

Each Bulldozer "core" consists of two dedicated integer ALUs, each having its own scheduler and L1 cache, and a single floating-point unit capable of executing two floating point threads. Each core has shared fetch, decode, and L2 cache hardware. According to AMD, the shared components help reduce power consumption and die space while the dedicated hardware helps increase performance and scalability. Unlike Intel, which made a trade-off to do more per clock cycle rather than pushing clock speeds, AMD intended Bulldozer to bump clock speeds to 4GHz and beyond.

Unfortunately for AMD, Bulldozer appears to miss the mark in almost every way. Clock speeds are lower than anticipated, power consumption is high, and most importantly of all, performance in most benchmarks leaves a lot to be desired.

The eight-thread 3.6GHz FX-8150 CPU, the current top-end processor in the line, has a base clock that tops previous Phenom II processors by 16 percent, far lower than the anticipated 30 percent boost. It can run half its cores at a much higher turbo frequency, as high as 4.2GHz, but only under certain workloads and only for short periods before hitting its thermal ceiling.

In typical desktop scenarios—productivity, content creation, and gaming—the Bulldozer usually performs worse than Intel's slightly cheaper four-core, four-thread Core i5-2500K. Sometimes much worse, dropping behind the cheaper-still i5-2400. These workloads are increasingly becoming multithreaded, but only to a point; most of them still cannot fully exploit an eight-thread processor, and have a much greater dependence on single-threaded performance—at which Sandy Bridge excels.

In benchmarks that could really take advantage of the FX-8150's eight threads, the AMD processor usually fared better, but for the mostpart, it still wasn't class-leading. It was competitive with the i5-2500K, but Intel's more expensive four-core, eight-thread i7-2600K still tended to have the edge.

Worse still, in a number of benchmarks the new processor did not even beat AMD's previous top dog, the Phenom II.

AMD's move to a 32nm process was also supposed to increase the performance-per-watt ratio, but the news is bad here, too. The FX-8150 has a thermal design power of 125 W (for short periods it can spike above 125W, but the long-term average is capped at that level), whereas the i5-2500K and i7-2600K are both rated at just 95W. With Intel's processors being as fast or faster than the Bulldozer, "performance per Watt" certainly isn't one of Bulldozer's strengths.

Server-oriented workloads may fare better. Server workloads are typically mulithreaded and integer heavy; these should be a good match for Bulldozer's abundant dedicated integer hardware and eight-thread design. The processor die contains plenty of high-bandwidth HyperTransport hardware for multiprocessor setups (which is actually disabled on desktop parts). However, this is speculation at present; server parts are yet to be benchmarked, and for the time being have all been earmarked for supercomputer customers.

On the desktop, the current incarnation merely provides ho-hum performance for many tasks and decent performance in certain highly threaded workloads. The processor costs more than a roughly equivalent Intel part, and thanks to the high power usage, the processor will cost more to run than a roughly equivalent Intel part. Anyone building or buying a new PC has little reason to even consider Bulldozer. Further, the inability to consistently beat Phenom II means that even existing AMD users with AM3 motherboards might be hesitant to upgrade by dropping in a new processor.

AMD is planning to ramp the performance of successive Bulldozer-based processors up by 10-15 percent every year. But next year's Ivy Bridge update is set to bring significant power savings and performance improvements with Intel's move to 22nm tri-gate transistors. AMD may have been able to push Bulldozer to record breaking speeds, but it will have a much harder time keeping the platform competitive with Intel over the next few years without significantly improving the performance per watt or improving yields with Global Foundries enough to significantly drop its price.